Recent small apparatuses such as a portable telephone have a built-in image taking unit for shooting an image of a subject. As a user of such a small apparatus need not carry a digital camera or a video camera in order to take a photograph, he/she can always take a photograph easily. Also, such a small apparatus generally is provided with a data communications capability using radio or infrared rays. Thus, such a small apparatus has another advantage that a user can send image data to another portable telephone or a personal computer immediately after shooting.
However, the image taking unit mounted in the small apparatus such as a portable telephone is so small that the unit has constraints on size of a charge-coupled device (CCD) and a lens as well as on space for accommodating them in the unit. As a result, the small image taking unit has a difficulty in exhibiting a photographic function and an image quality equal to those of a digital camera. Therefore, a use of the image taking unit is limited in photography in which a high image quality is not required, for example, a photograph substituted for a memorandum and a photograph for an image to be displayed on a stand-by screen of a portable telephone.
In this respect, the recent development of a small CCD with many pixels and a small lens corresponding to the type of CCD has improved an image quality of a photograph taken by using the small apparatus. The remaining problem is to improve the shooting function. In order to address this problem, it is desired to provide zoom and autofocus functions in the small apparatus which are ordinarily mounted in a digital camera.
In general, these functions are realized by moving a lens in the direction along the optical axis using rotation of a motor. In many cases, an electro-magnetic motor in which a rotor is rotated by a magnetic field is used as a motor to move a lens. But, an electro-magnetic motor consumes a lot of electricity and is rather large as a driving source. Accordingly, mounting an electromagnetic motor on the small apparatus results in significant increase in size and weight of the small apparatus. Moreover, mounting an electromagnetic motor requires an amount of electricity large enough to drive the electromagnetic motor in addition to the electricity used for other general photographic functions. Therefore, it is difficult to mount zoom and autofocus functions by using an electromagnetic motor on a portable telephone and the like, which is required to be small and lightweight.
Regarding this point, a photographic apparatus in which an actuator making use of piezoelectricity is used instead of an electromagnetic motor in order to move a lens is disclosed in Patent Documents 1 and 2. Also, in Patent Documents 3 and 4, a basic structure of an actuator making use of piezoelectricity is disclosed.
FIG. 1 is a schematic diagram of an ultrasonic actuator making use of piezoelectricity, and FIG. 2 is an explanatory diagram of a principle on which the ultrasonic actuator operates.
As shown in FIG. 1, the ultrasonic actuator 10 has a piezoelectric device 11, an elastic vibrating object 12, a rotor 13, a holding member 14, a spring 15, and a press plate 16. The piezoelectric device 11 vibrates in response to application of a voltage and the elastic vibrating object 12 is distorted by the vibration of the piezoelectric device 11. The rotor 13 rotates due to the stress of the distortion of the elastic vibrating object 12. The holding member 14 has a role of holding the piezoelectric device 11 and the elastic vibrating object 12. The spring 15 serves for pushing the elastic vibrating object 12 toward the rotor 13, while the press plate 16 serves for pressing the spring 15 toward the elastic vibrating object 12.
As shown in FIG. 2, the elastic vibrating object 12 is sandwiched between the two piezoelectric devices 11a, 11b. When an alternating voltage is applied to each of the two piezoelectric devices 11a,11b with the same phase, these piezoelectric devices 11a,11b expands and contracts in the same direction. The expansion and contraction cause distortion of the elastic vibrating object 12, whose tip is pressed toward the rotor 13. The tip of the elastic vibrating object 12 is driven in such a way that it moves depicting an ellipse. By the movement of the tip of the elastic vibrating object 12, the rotor 13 rotates in the direction of an arrow A in FIG. 2.
In an image taking unit provided with an ultrasonic actuator making use of piezoelectricity such as the above one, it is possible to drive a lens with less electricity than an electromagnetic motor. Moreover, it is also possible to realize a lightweight image taking unit which can move a lens quietly.                [Patent Document 1] Japanese Patent Application Publication No. 2004-294759        [Patent Document 2] Japanese Patent Application Publication No. 2004-294580        [Patent Document 3] Japanese Patent Application Publication No. 2005-218179        [Patent Document 4] Japanese Patent Application Publication No. 2003-199371        
Meanwhile, as forms of distortion of an elastic vibrating object, there are known three kinds of vibration, that is, a longitudinal vibration originated from expansion and contraction of the elastic vibrating object, a bending vibration originated from waving of the elastic vibrating object, and a combination vibration made of the two vibrations combined together. In order to realize fast rotation of a rotor, it is desirable to cause the elastic vibrating object to have a combination vibration. However, according to the techniques disclosed in Patent Documents 3 and 4, alternating voltages need to be applied to each of plural piezoelectric devices with different phases to cause the combination vibration of an elastic vibrating object. Therefore, these techniques have such a problem that the voltage control becomes complex.
In addition, a conventional ultrasonic actuator making use of piezoelectricity requires a pressing structure having elements such as the spring 15 and the press plate 16 in order to press the elastic vibrating object 12 toward the rotor 13, besides the holding member 14 that holds the elastic vibrating object 12. Although the additional pressing structure is not directly related to the distortion of the elastic vibrating object 12, it requires space equal to that required by other parts of the ultrasonic actuator, thereby preventing a reduction in size of an ultrasonic actuator.
Accordingly, in order to mount an ultrasonic actuator making use of piezoelectricity in the above-described image taking unit for a portable telephone and the like, the ultrasonic actuator is required to be smaller in size and the voltage control is required to be further simplified.
The above problems are not limited to image taking units, but they occur in any field in which an ultrasonic actuator driven by application of a voltage is used.
In view of the above circumstances, it is an object of the present invention to provide a small ultrasonic actuator with easy voltage control.